Generally, there is a need for methods to bond dissimilar materials. There is a great need for such methods in applications where the bonded dissimilar materials will be subjected to thermal or pressure variations, low pressure/vacuum, or other stresses that tend to separate the bonded materials.
As an example, each of the three major Earth Observing Satellite (EOS) observatories (Terra, Aqua, and Aura) has passively cooled cryogenic instruments (e.g. MODIS, AIRS, and TES) with radiators that point to deep space. These radiators must be shielded from Earth's albedo with a shade panel extending 1-2 m from the radiator beyond the penumbra of the spacecraft from the Earth as well as the sun. The outside of the shade is coated with a low absorptance, high emittance white paint to prevent the shade from getting too hot. The inside surface of the shade must have low absorptance and low emittance as well as high specularity to prevent the solar radiation that comes over the top of the spacecraft from reflecting back onto the radiator surface. As the spacecraft orbits Earth, it passes in and out of the Earth's shadow and experiences wide swings in the external thermal environment. The coatings inside and outside the shade must maintain their properties over this range of temperature.
Mass constraints for these missions have driven the use of low mass, high stiffness composite sandwich panels in place of the more traditional aluminum panels. These composite materials may be composed of multiple plies of either fiberglass or carbon fibers in an epoxy matrix sandwiched with honeycomb that results in a very low thermal expansion. Coating processes and materials that were previously successful with high thermal expansion aluminum panels do not work well with the newer low expansion composite structures.
In the space environment, earth orbiting satellites may be exposed to substantial temperature variations as they traverse their orbits, spending a portion of their orbit in direct sunlight and a portion of their orbit shielded from the sun by the earth. To maintain acceptable thermal control under such conditions, satellites are often coated with specular coatings that provide desirable properties of absorbance and emittance when properly bonded to a spacecraft structure to maintain spacecraft components within acceptable temperature ranges. It may also be desirable to bond RF reflective coatings to spacecraft structures to create a light weight antenna for communications. There thus is a need for methods to apply or bond such specular or RF coatings to spacecraft structures, where the coatings have dissimilar material properties, such as thermal coefficients of expansion, from the structures. Conventional methods exist for bonding materials, but such conventional methods do not reliably maintain attachment between dissimilar materials when exposed to substantial thermal or pressure variations, low pressure/vacuum or other stresses that tend to separate the bonded materials.
Specular surfaces have been used for thermal control on spacecraft for many years. One method of creating a specular coating is coating the outer aluminum surface with lacquer to make it smooth and then vapor depositing aluminum onto the lacquer surface to make it shiny/specular. Lacquer also has a high thermal coefficient of expansion that works well with aluminum. However, tests to demonstrate this on carbon fiber composite panels have shown that the high differential thermal coefficients of expansion between the lacquer and the composite panel showed cracks in the lacquer that increased in number and depth with thermal cycling to the point of breaking apart the fibers in the first ply of the composite face sheet.
Another method is to deposit the aluminum directly onto the face sheet. The face sheets are laid down on glass and consequently have a very specular surface, although the morphology of the top ply of fibers shows through and is easily detectable with optical measurements. This can be acceptable where the specularity requirement is less than 90%, but may not be suitable when higher specularity is required. However, when applied to carbon fiber face sheets, the aluminum film can be destroyed by galvanic corrosion between aluminum and small areas of exposed carbon fiber.
Yet another alternative for achieving a specular surface on a carbon fiber composite face sheet is a reverse coating or replication process. First a mold release film is applied to a tool plate, such as glass, followed by vapor depositing a coating of aluminum. Epoxy then is applied to the aluminized surface followed by the composite panel. After curing the composite panel, the tool is pulled away from the coated panel. This method is not easily repeatable and can be very expensive. In testing this method, sometimes all or part of the aluminum stuck to the glass of the tool plate, leaving blank spots on the panel. In order to achieve the required specularity, the epoxy was applied in a thick coating. This sometimes resulted in the entire layer peeling off during thermal cycling. With this process, if the coating process was not successful, the panel had to be scrapped, as there was no way to remove the epoxy layer without damaging the face sheet.
Still another method to laminate/bond a film to a substrate involves the use of a pressure sensitive adhesive. While the pressure sensitive adhesive is capable of withstanding the vacuum, it is temperature limited by the softening point or the glass transition temperature (Tg) of the adhesive. The glass transition temperature is the temperature below which molecules have very little mobility. When the adhesive is subjected to a solar simulation test, the film attached with pressure sensitive adhesive shows bubbles/blisters when the substrate reaches or exceeds the Tg of the adhesive. The change in morphology caused by the bubbles or the blisters is unacceptable for any lightweight structure requiring a thermally stable coating that does not delaminate, blister, or peel, such as for solar arrays.
Conventional methods to adhere specular or RF reflective coatings to spacecraft structures thus have been found to delaminate, blister, or peel when exposed to the extreme conditions of space, which can reduce the specularity or RF reflectiveness of the coating and undesirably alter the thermal or RF properties of the structure/coating. Accordingly, a need exists for improved methods to bond dissimilar materials.